An image forming apparatus based on electrophotography, such as a copying machine, a facsimile, a printer, and the like, uses two-component developer. The two-component developer is constituted of toner (dye powder) and carrier (magnetic powder) and is contained in a developing tank. Only toner out of the two-component developer is used to form an image. In order to obtain a clear image in a stable condition by using the two-component developer, it is important always to maintain a mixing ratio of components in the best condition. Therefore, this type of image forming apparatus is provided with a toner density sensor by which the mixing ratio of components is maintained in a constant value. Toner is supplied from a toner cartridge to the developing tank according to an amount of toner consumption.
For example, Patent Document 1: Japanese Unexamined Patent Publication 44066/1992 (Tokukaihei 4-44066) (published on Feb. 13, 1992) discloses an image forming apparatus that detects toner density in changing developer, and controls (corrects) toner density so that the detected value is set as standard toner density.
In general, an image forming apparatus includes a plurality of printing modes different from each other in a printing velocity. In these printing modes, the printing velocity is changed by changing the number of rotation of a photoconductor drum and process velocity. As a result, stirring velocity of two-component developer contained in a developing tank changes according to the printing modes.
However, the image forming apparatus disclosed in Patent Document 1 performs the same toner density control when the printing mode (printing velocity) changes. As a result, it is impossible to detect toner density with high accuracy, so that it is impossible to stably supply toner to the developing tank.
Patent Document 2: Japanese Unexamined Patent Publication 72660/2002 (Tokukai 2002-72660) (published on Mar. 12, 2002) discloses an image forming apparatus in which an input voltage of a toner density sensor is set to many values according to toner densities. With the arrangement, the input voltage is varied (switched) according to the printing velocity and toner density, so as to realize detection of toner density with high accuracy.
However, toner density control with high accuracy cannot be performed only by switching input voltages corresponding to the printing velocities, as described in Patent Document 2.
Namely, the image forming apparatus disclosed in Patent Document 2 adjusts the toner density sensor by using typical printing velocity out of the printing velocities. However, it is proved that when an image is formed at printing velocity different from printing velocity used in adjustment, detection of toner density with high accuracy cannot be maintained.